Thenzsea, I assume you are not talking about a Beneteau 40.7 (designed by Farr) since they don't have running backstays. You are talking about a 1990's era Farr IMS 40 which does indeed have running backstays. Is that correct?

If that is the case, what you want is a single permanent backstay on centerline like a traditional backstay. The problem is that if you may have clearance problems with the roach of the main. The problem can be reduced by installing a batten at the masthead that will lift the backstay up when it is eased to help the sail clear the stay. It is sometimes referred to as a "flicker". Depending on the cut of your mainsail it may only be a problem in light air.

I can't think of anywhere else you could install a permanent stay where it would be at all effective. I suppose another possibility could be if your existing backstays can clear the main, then you can make them permanent, and just have running checkstays alone.

That is the problem with race boats that depend on running backstays and checkstays to keep the mast up! There is no room for mistakes!

I would talk to your local rigger about installing a permanent backstay, or other options. rather than doing it yourself. As you probably know, dropping your mast again is expensive and dangerous!